Photocell



Dec. 2s, 1937. F, SCHRTER 2,103,498

PHOTOCELL Filed March 8, 1933 Y BOY@ I ATTORNEY Patented Dec. 28, 1937 ATEy T QFFICE PHOTOCELL Fritz Schrter, Berlin, Germany, assignor to Telefunken Gesellschaft fr Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany Application March 8, 1933, Serial No. 660,008 In Germany March 11, 1932 7 Claims.

The present invention is particularly directed to a photoelectric cell or photo-tube with amplifying action.

If in a triode tube of standard type an electron current which is kept constant by the action of special means, e. g., working inside the range of saturation, is caused to flow through the meshes of a grid subjected to a marked positive biasin what is known as a retarding field circuit scheme (oscillating electrons), it is found that the electrons experience retardation if the potential of the retarding electrode is lower than that of the grid. If the potential of the retarding electrode is made equal to that of the lament, or slightly more negative than the latter, no electrons will finally reach the retarding anode any more; in-

deed, they will becoinpelled toreturn by the latter.

More recent investigationshave demonstrated that the tubes, in the neighborhood of a state as mentioned, exhibit a very marked control or revspouse sensitiveness even to slight voltage fluctuations in the retarding plate. This is probably because of the fact that, in the presence I" constant aggregate current, similarly large and opposite uctuations of the Vcurrent ilowing to the grid or the retarding plate are occasioned, the grid current component being able t0 cause considerably amplified alternating voltages across the terminals of a high external resistance.

The present invention is concerned with a sys- 1 .tem of the kind hereinbefore mentioned. One of the objects of the present invention is the creation of a highly sensitive photoelectric cell with amplifier action. With this end in View the retarding electrode is rendered photoelectrically responsive by ways and means well known in the prior art and is so mounted that it will be struck by impinging light. The constant stream of electrons issuing from the cathode, in a way as outlined in the preceding paragraph, thus reaches across the grid electrode, which is positive in reference to the cathode, into the eld existing between the grid and the photoelectrically active retarding electrode. If the potential of the retarding electrode in reference to the other electrodes is suitably chosen, there will then arise directly in iront thereoi a negative space-charge which, if the system is adjusted in a deiinite Way, will be stationary.

This space-charge layer built up of returning or reversed electrons in the area around the retarding electrode is greatly responsive to control actions occasioned by electrostatic influences. Such action, according to the present invention, is produced by the electrons emerging from the (Cl. Z50-41.5)

activated retarding electrodes under the action of light, and these electrons become added to the cloud of electrons located in iront of the issuing surface so that, in the presence of unvaried direct current potential of all electrodes, they cause a change in the current flowing to the grid, such change lasting until the new state of balance has been attained and until the space-charge has become stable or stationary again. The voltage variation effected across the terminal of a high outside resistance being in series with the grid electrode controls the ampliiier provided below. n the case of intermittent illumination, alternating voltages of Vappreciable amplitude are generated.

Where a heated cathode is employed it is advisable to use a source of electrons which is sufficiently emissive also at low temperatures in order to avoid harmful stray illumination ofthe activated retarding electrode, e. g., an oxide cathode. The electrodes most suitably are so mounted that the volume of light rays reaching the retarding electrode from the cathode will be cut down to a minimum.

The drawing shows schematically an exemplified embodiment of the basic idea of this invention and may serve to explain the latter in more detail. Inside the tube designed at I the indirectly heated oxide filament is shown at 2 While the grid electrode is represented by numeral 3 and the photoelectrically active retarding electrode by numeral t. The latter may consist, for instance, of an extremely thin deposit of caesium upon oxidized silver foil or sheeting, of a hydrogenated potassium coat or some other suitable substance. It will be .understood that the insulation in reierence to the other electrodes must be adequately high. The voltage variations arising across the external resistance 5 contained in the circuit of grid 3, if necessary, are thereupon fed to the grid of another amplier tube.

By an arrangement of the kind as herein disclosed and illustrated, it is possible to realize in the neighborhood of zero potential of the retardying electrode il, compared with the cathode 2, a

very readily controllable and responsive action of the photo-electrons. In other Words, it is possible to obtain a very steep alteration oi the distribution relations of the saturation current over the electrodes 3 and d; The best plan is to adjust conditions to the steepest point or change in the grid current. What is presupposed is that the aggregate current, by operation inside the saturation range, or by other ways and means, is kept unvaried. With this end in View, the biasing voltage of the grid 3 in reference to the cathode 2 must be chosen so high that, according to the state of incandescence of the latter, all of the electrons given oi by the surface of the same will be drawn olf. The amplification factor or gain of the arrangement will depend upon the relationship between the external resistance 5 and the inner resistance of the retarding electrode path between electrodes 2 and 4. Inasmuch as the latter is of an order of magnitude of 1000 ohms, while the external resistance, in the presence of very high frequencies of illumination may still bea very high multiple of the aggregate internal resistance, it will be seen that not only very high gains, but also uniform ampliilcations inside wide frequency bands are securable, a cir cumstance that is of great value in optico-electric conversions in talking film, in picture transmission or telegraphy, and in television work.

The invention is not restricted to the use of triode tubes of a special type, although these have been exclusively taken into consideration in what precedes. As a matter of fact, for the identical reasons, similarly as with normal or standard electron tubes, additional grids, such as screen-grids, space-charge grids, collector grids, regulator grids and the like, may be used with a View to improve the action of the arrangement.

Having now described the invention what I claim is:

1. In a system for amplifying photoelectric currents, an electron tube comprising a heated electron emitting electrode, a grid electrode, and a photo-sensitized retarding electrode, a resistor and a source of potential for maintaining the grid electrode positive with respect to the heated emitter, and means for maintaining the photosensitive retarding electrode at a predetermined negative voltage relative to the heated emitter so that upon illumination of the photosensitized electrode magnified voltage variations are produced across the said resistor element.

2. In a system for amplifying photoelectric currents, an electron tube comprising a heated electron emitting electrode, an electrostatic control electrode and a photo-emissive retarding electrode, a resistor and a source of potential connecting the electrostatic control electrode and the heated emitter, said control electrode being maintained at a positive potential with respect to said heated emitter, and means for maintaining the photo-emissive retarding electrode at a predetermined voltage less positive with respect to the control element so that upon illumination of the photo-emissive electrode magnified voltage variations are produced across the said resistor element.

aosfies 3. In a photoelectric cell having a photosensitive electrode, an electrostatic field control electrode, and an output circuit, the method of amplifying photoelectric currents which comprises the steps of producing a stream of electrons, producing an electrostatic electron accelerating field, producing a contiguous electrostatic electron retarding iield, directing the produced stream of electrons successively through the produced accelerating and retarding elds, collecting electrons at the boundary between the contiguous elds, conducting the collected electrons to the output circuit and modifying, in accordance with impinging light energy, the magnitude of the retarding eld whereby the flow of said electrons is increased in said output circuit.

4. In a photoelectric tube, the combination of a photosensitive anode, an electrostatic control element and a thermionic source of electrons, means including a serially connected resistance for maintaining said control element at a positive potential with respect to said source of electrons, means for maintaining said anode at a potential less positive with respect to said control element, and means to control the flow of electrons through said resistance in accordance with light impinging on said photosensitive anode.

5. In a triode thermionic tube operated as a dynatron wherein is provided an electron emitting cathode, a grid and a photosensitized anode, the method of controlling the amplification of said tube comprising the step of subjecting the photosensitizedY anode to light to vary the electron emission therefrom.

6. In a photocell control system wherein there is provided an electron source and a light initiated electron source, the method of controlling the flow of electrons which comprises successively subjecting the electrons from the first named source to an electrostatic accelerating eld and a contiguous electrostatic retarding eld, and photoelectrically altering the electrical retarding field to vary the density of electrons at the boundary between the contiguous iields.

'7. The method of signal control which comprises the steps of producing two electron streams, directing each of the produced streams in opposite directions toward an output electrode, photoelectrically controlling one of the produced streams to vary its intensity, and subjecting the other produced stream to the control of the photoelectrically controlled stream to vary the total number of electrons reaching the output electrode.

FRITZ sCHR'rER. 

